Many composite articles, for example, composite structural panels such as the center shear deck of a helicopter, require stiffening members to enhance the structural strength of the main structural panel. To ensure that such composite articles are functional for their intended purpose, the stiffening members must be rigidly secured to the main structural panel. In addition, the composite manufacturing technique must produce composite articles having smooth surface finishes, i.e., the fabrication technique must not induce distortions or defects in the skin surfaces of the main structural panel or the stiffening members.
Composite structural articles having stiffening members have been fabricated, where possible, so that the stiffening members are all disposed on only one side of the main panel. This technique greatly simplifies the manufacturing process and ensures the suitability of the finished composite articles. Any one of several techniques may be utilized to form composite articles having stiffening members disposed on only one side of the main structural panel. Prepreg laminates comprising the main structural panel may be layed-up on a rigid mold, which provides the required surface finish for the main structural panel. The prepreg plies comprising the stiffening members may be layed-up on the prepreg laminates and maintained in place by means of a preformed mold, a semi-rigid membrane, or mandrels and a vacuum bag. The assembled mold may be placed in an autoclave and subjected to a cure process wherein the stiffening members are simultaneously integrally bonded and co-cured in combination with the main structural panel. The preformed mold, the semi-rigid membrane, or the mandrels provide the required surface finish for the stiffening members. A more detailed description of the fabrication technique utilizing a preformed mold may be had by reference to U.S. Pat. No. 4,492,607.
While the foregoing fabrication techniques are suitable for the fabrication of composite articles having stiffening members disposed on only one major surface of the main structural panel, a problem arises when stiffening members are required on both major surfaces of the composite article. One fabrication technique for composite articles having opposed stiffening members involves the fabrication of a composite article having stiffening members on one major surface as described in the preceding paragraph. Precured stiffeners may then be secured to the other major surface by drilling holes through the corresponding skin of the main structural panel and utilizing composite fasteners to secure the precured stiffeners to the surface thereof. In addition to the additional cost and time involved in the labor intensive securing operation, a serious deficiency with this technique is that stress concentrations are introduced in the composite article at the stiffening member securement sites.
Another fabrication technique for producing composite articles having opposed stiffening members involves the fabrication of a composite article having stiffening members on one major surface as described hereinabove. Precured stiffening members are then attached to the other major surface using a thermoset adhesive, and placing the assembled composite article in an autoclave to cure the thermoset adhesives. One problem with this method is that the secondary bonding process produces weaker composite articles as compared to integrally bonded or co-cured composite articles. In addition, the additional manufacturing steps increase the cost of such composite articles, cost being directly proportional to the number of manufacturing operations and cure cycles necessary to produce the finished composite article.
A need exists for a fabrication technique and molding apparatus for forming composite articles having opposed stiffening members integrally bonded to the main structural panel. The fabrication technique/molding apparatus should provide for simultaneous co-curing of the opposed stiffening members with the main structural panel to form the composite article.